A variety of techniques have been developed for the production of ceramic particles which involve the precipitation of a precursor of the powder from an aqueous solution containing the desired cations of the ceramic. In many of these techniques, the solution is mixed with a reagent which will precipitate the cations in the form of easily reducible compounds, such as hydroxides, carbonates, oxalate, etc. The precipitates are separated from the liquid and sintered to reduce them to the respective oxides. A technique, which is particularly advantageous in developing ceramic particles in the micrometer size or less, is disclosed in co-pending Canadian patent application Serial Number 544868-9, filed 19 Aug. 1987 of which one of the two inventors is also co-inventor of this application.
Other techniques for preparing ceramic powders are disclosed in French patent 2,054,131. The patent disclosed the emulsification of an aqueous solution of the metallic salts which form the ceramic. The emulsion is treated to remove the liquid and calcine the resultant solid phase to produce the ceramic particles.
Considerable attention has also been given to the development of micron size particles for use in biological treatments. A particular area of interest is the development of magnetic particles agglomerated or individually coated with materials to which biological substances can adhere. Examples of magnetic particles for use in this manner are disclosed in U.S. Pat. Nos. 3,330,693; 4,152,210 and 4,343,90l. European Patent application 176,638 published Apr. 9, 1986 also discloses the use of magnetic particles for the immobilization of biological protein. Several of these patents contemplate coating of the magnetic core with a polymeric material, or agglomerating several particles in a suitable polymer such as disclosed in U.S. Pat. No. 4,343,90.
The use for magnetic materials in the biological field continues to increase, hence an increased demand for superior materials. Consider, for example, the use of such particles for immobilizing enzymes or antibodies. Separation of such materials from other non-magnetic solids by the use of a magnetic field permits separations and concentrations which would be otherwise difficult or even impossible to perform. Besides allowing separation of the support from suspended solids in the process liquids, the ease and power of magnetic collection permits the use of very small support particles. In turn, this allows the use on non-porous particles, while still retaining a reasonable specific area for enzymes or antibodies. Another advantage of such magnetic materials is their potential use in a magnetic stabilized fluid bed, thereby presenting further options in continuous reactor systems.
From the noted patents, a variety of magnetic materials have been used in the preparation of magnetic supports matrices including iron, nickel, cobalt, and their oxides as well as composite materials such as ferrites. However, such supports suffer from some disadvantages. First, metal ions from uncoated metal or metal oxide surfaces may irreversibly inhibit some enzymes, particularly when the enzyme is attached directly to the metal surface. Methods have been devised to attach the enzyme to the inorganic material with the aid of intermediate crosslinking agents and/or to coat the magnetic material with organic coatings as noted in U.S. Pat. No. 4,152,210.
Coating of magnetic material with inorganic coatings has also been proposed. U.S. Pat. No. 4,343,901 describes a magnetic support matrix comprising a porous refractory inorganic oxide, through the interior of which are dispersed particles from about 0.05 micron to about 0.5 millimeter of ferromagnetic materials, said oxide being impregnated with a polyamine cross-linked with an excess of a bi-functional reagent so as to furnish pendent functional groups. The refractory inorganic oxide, which may be obtained by a sol-gel technique, is calcined before use. Ferro-magnetic materials above 0.05 micron in size are not superparamagnetic and therefore exhibit permanent residual magnetism. Furthermore, the coatings proposed do not appear to be continuous and as a result would not prevent losses in enzyme activity.
Coated magnetic particles have been also devised for various alternative uses. GB 2064502 describes a method of making coated magnetic particles, for use in ion-exchange resins, filter aids or absorbents, by precipitating chromium hydrogel onto magnetic particles from 0.05 to 5 microns in diameter and which are therefore not superpara-magnetic. The proportion by weight of magnetic particles in the coated magnetic particles is at least 50%, generally 90 to 98%.
JP-A-6364308 describes magnetic fluids containing permanently suspended particles comprising ferromagnetic material dispersed in a heat-resistant inorganic oxide.